1. Field of the Invention
This invention relates generally to electrical protective devices and, more particularly, to a device for interrupting electrical current to an electrical appliance and the like for excess values of electrical current.
2. Description of the Prior Art
When most electrical appliances are connected to the usual wall receptacle they are connected electrically to a power line without any protection against surges of electric current which might seriously damage them. Generally, the only safety devices in these power lines are fuses or circuit breakers which are adapted to burn out or open when the lines that they service sense a current overload. These devices are not adequate to protect appliances since a current overload which might be less than required to open the circuit breaker or burn out a fuse might still be great enough to damage the appliance. Typically, these current overloads result from voltage surges caused by lightning striking the structure in which these appliances are housed, or by striking an exposed power line.
A solution to this problem is the isolator presented by James F. Worthington in U.S. Pat. No. 3,539,961. The isolator had a male plug member and a female receptacle electrically interconnected by fusible wire. An arc plate, connected to a third prong, is disposed adjacent the male prong members of the isolator so when current in excess of a predetermined value flows through the device, the fuse wire is melted or burned out and the current is carried by the arc plate to ground, thereby effectively isolating the appliance and saving it from damage. In actual practice, the fusible wire did not consume itself rapidly enough to prevent excess, damaging current from reaching the appliance. Also, the fusible wire was not self-restoring and needed replacement by a technician.
Another solution to this problem is presented in my pending application, INTERRUPTER, Ser. No. 644,422, filed Dec. 24, 1975, wherein a voltage responsive resistive element, e.g. varistor, carbon pellet arrester, gas ionization tube, is used to short circuit the female receptacle when a power surge condition occurs. This is an improvement over the isolator of Worthington, in that it is faster and self-restoring; however, the varistors and carbon pellets when heated by high current flow are relatively slow to regain their normal operating characteristics, and the gas ionization tube does not extinguish until voltage has fallen to a low level. Thus, there exists a need for a low cost current interrupter which is self-restoring and faster acting.